Liquid substances, e.g. concentrated fruit juice or pulverulent materials, are conventionally conveyed in rigid containers such as metal or plastic vats.
This method of transport has two main disadvantages—the bulk of the empty vats when conveyed before or after conveying the liquid, and the weight, particularly of metal vats.
It has therefore been proposed to remedy the disadvantages by using flexible envelopes or liners adapted to convey liquids and positioned either in movable rigid containers or in likewise flexible containers, mainly envelopes of woven fabric.
A flexible envelope associated with a flexible container is the most radical solution and also the most efficient as regards reduction of weight and bulk when empty. It is also the most advantageous economically.
However, a flexible envelope associated with a flexible container has a disadvantage compared with a vat or a rigid container.
During filling or emptying, the valve has to be secured to the drain or fill pipe, which involves moving the pipe when screwed to the valve body. Rigid containers enable the valve to be firmly secured to the container, thus ensuring a fixed, invariable position of the valve even and particularly when subjected to a screwing operation. The problem is precisely the same with a bayonet fastening.
On the other hand a flexible container cannot be secured so as to fix the valve in an invariable position. The essential operation, i.e. connecting or disconnecting the valve to or from the fill or drain pipe, is therefore hazardous since the inner envelope is very sensitive to twisting or rolling and may break by tearing.